The burglars climbed the fence and forced their way through a second-story window into the Franklin Mineral Museum early Monday morning, investigators said. They rappelled down to the main floor to steal the gems, stones and minerals.

An alarm was triggered, police told the New Jersey Herald , but an officer who went to the museum found nothing amiss. The theft was discovered when an employee arrived to open for the day.

Police say the thieves caused substantial damage.

Investigators believe one or more of the suspects was hurt and are asking the public to be on the lookout for anyone with an unusual laceration.

The Franklin Mineral Museum tells the story of the Franklin zinc mine in Sussex County. Its mineral exhibit houses about 5,000 items, according to the museum's website.

In 2011, more than $400,000 worth of gold was stolen from the Sterling Hill Mining Museum in Ogdensburg, just a few minutes away from the Franklin museum. The gold was never recovered.